1. Field of Invention
The invention relates generally to building construction and more particularly to floor joist systems used in building construction, especially residential construction.
2. Background Art
Floor joists used in residential and some smaller commercial building construction are typically made of wooden 2 inch by 10 inch planks or beams of engineered wood having an I shaped cross section with a 2 inch wide flange and a 10 inch height which extend longitudinally between opposite side walls of a house or its foundation, resting on the top surfaces thereof, providing support for the floor of each individual building story. In residential construction, the subfloor adjacent the joists is usually 3/4 inch thick plywood. Generally, wooden joists are limited in their length or span to about 14 or 16 feet, at which point they must be supported from beneath, usually by an intermediate load bearing wall or a girder (or header) extending in directions perpendicular to the joists, the girder itself perhaps partially supported by a support post or column and/or the side walls between which the girder extends. The girder may be comprised of a plurality of 2 inch by 12 inch planks disposed side by side to provide additional thickness or may be an engineered wood beam or steel I beam. Other types of joists and/or girders used in larger commercial buildings, which often have poured concrete floors, include steel or iron I beams which have integrated flanges and webs or which are assembled from components, as disclosed in U.S. Pat. Nos. 669,639 (Hessel et al.), 4,151,694 (Sriberg et al.) and 3,800,490 (Conte). Concrete floors and metal joist systems are not generally used for residential construction due to the higher costs involved and their not being readily adapted to conventional housing designs.
A plurality of parallel floor joists laid out across the area bounded by the supporting side walls provides a series of generally coplanar surfaces to which the subfloor is attached, usually by adhesives and nails or screws. The attachment of the subfloor to the topmost surfaces of the joists prevents the joists from moving, although it is common to provide braces therebetween to stabilize them. The joists and girders are oriented so as to expose their maximum bending moments against the loading of the above floor; this normally entails setting the joists on the side walls in an upright manner upon one of their shorter rectangular sides or their I beam flanges, the opposite short rectangular side or I beam flange abutting the lower surface of the supported floor. A pocket or recess provided in the girder bearing side wall provides a surface upon which the girder rests, the surface disposed a distance below the top of the side wall somewhat equivalent to the height of the girder. This arrangement allows the bottom most surface of the joist to rest on the top surface of the side wall and the girder. Disposing the girder as such and disposing the joists thereupon, however, compromises the ceiling height of the below room at least partially or otherwise forces the floor of the above room to be higher. Furthermore, the below room ceiling height may be further compromised, at least locally, by pipes, wiring or ventilation ducts routed below the girder.
Joists are usually transversely spaced in a parallel fashion at fixed distance from each other in accordance with the weight bearing characteristics of the materials used and the designed building load requirements. Typically, in residential construction, wooden joists of either the plank or engineering beam variety are spaced 16 inches on center. Wooden plank and engineered wood floor joists are maintained in their upright positions, i.e., kept from falling over, and spacing relative to one another by lateral braces which do not interface the lower surface of the floor or support or help distribute its weight. Steel I beam type floor joists such as used in commercial building construction may likewise be maintained in position by braces interconnected with the webs thereof, although the wide bottom flange of most steel I beams is sufficient to prevent its inadvertently falling over.
Wooden floor joists of the plank or engineered beam variety are generally limited to 14 or 16 foot spans between supports and 16 inch on center spacing relative to one another, requiring many joists and supporting girders be provided in a house of conventional size and design, thus comprising an appreciable portion of the cost of required building material, particularly if the more expensive engineered wood beams are used. As a further result, plank or engineered wood beam floor joist systems are rather expensive in terms of labor because of the quantity of joists required to be installed. Moreover, wooden plank joists may be irregular, undesirably having crowns or cupping, sagging or bowing. Often, significant effort and cost are required to correct these conditions during construction or their effects after the building is completed. Engineered wood beam joists resolve many of these issues, but are rather more expensive than plank joists and have no appreciably greater load bearing capability.
Wooden planks, as lumber, are considered to be commodities, and thus their cost is greatly influenced by fluctuating market prices, which can make estimating future building costs more difficult. Engineered wood beams, comprised to a great extent of wood chips and more labor intensive to produce, are not so readily influenced, although they are generally more expensive.
There is a need for a floor joist system which is relatively stronger and less labor intensive than previous systems employing wooden plank or engineered wood beam joists, provides a consistently flat flooring surface, more efficiently uses vertical space and is not greatly influenced by commodity market price fluctuations.